Molybdenum Deficiency in Cannabis Plants

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Molybdenum deficiency cannabis
Author avatar By Parker Curtis
March 01, 2022
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Cultivating weed is an enjoyable and rewarding task, but experienced growers know that there may be a few setbacks. Molybdenum deficiency in cannabis is one example of what can go wrong. 

If your marijuana crops look worse for wear and the leaves are discolored, you might have a molybdenum deficiency in your plants. Weed displays several signs when it lacks the nutrient, and we’ll highlight them below. 

We’ll also discuss the role of the molybdenum nutrient, possible causes of a deficiency, and what to do if your crops have a shortage. Let’s dive in with a detailed definition.

Cannabis plant with brown leaves
Cannabis plant with brown leaves

What is molybdenum deficiency?

Before you can diagnose or treat molybdenum deficiency in your weed, you need to understand what it is. 

Molybdenum is a nutrient that has a vital role in the well-being of your crops. 

It’s a mobile micronutrient, and your marijuana crops only require a small amount to function optimally. If there isn’t enough molybdenum nutrient available to your plants, they change color and get brittle before dying. 

Fortunately, if you observe your crops closely, you’ll quickly detect signs of a molybdenum deficiency in cannabis. The sooner you realize there’s an issue, the faster you can treat it and save your plants. 

What does molybdenum do for plants? 

The molybdenum nutrient plays an important part in two enzyme systems that convert nitrate to ammonium. This role is vital so the crops can easily synthesize amino acids

As a result, molybdenum facilitates plant growth by optimizing the way the plants access certain micronutrients. 

Molybdenum deficiency in cannabis has a catalyst effect because it directly impacts the plant’s nitrogen use, which is essential for cannabis crops. It’s typically active in the roots and seeds. 

What causes molybdenum deficiency in weed? 

There are several reasons why molybdenum deficiency in weed occurs. Even though it’s a micronutrient that exists in small doses in your cannabis plant, it affects how efficiently the nutrients are absorbed and used. 

The most common culprit is an unbalanced pH. Excessive iron in the soil and cold temperatures also cause this issue.

Cold temperatures

The amount of the molybdenum nutrient present in soil decreases significantly when the temperature drops. The onset of symptoms is rapid, and you can try to avoid this by adjusting the heat levels in the growing environment.

Excessive iron in the soil

The molybdenum nutrient is available in high pH soil, whereas you can find iron more easily in soil with low pH levels. If there’s plenty of iron available, it probably means that the substrate is too acidic to contain an adequate amount of molybdenum.

Having too much iron in the soil ties in with incorrect pH levels.

pH levels

Most of the time, an unbalanced pH is the cause of molybdenum deficiency in cannabis. 

Marijuana flourishes in pH levels that range between 5.5–6.2. If this ratio is too high or low, your crops suffer, and signs like discoloration and twisted leaves begin to show.

An imbalance can even cause a molybdenum nutrient lockout, which occurs when the pH is too low, and the growing medium becomes extra acidic. It inhibits the roots from absorbing molybdenum and subsequently affects the plant’s overall health.

If your growing medium is soil, the ideal pH range is between 6 and 6.5. Try not to let it fall below 6. If you’re cultivating in a soil-less medium like hydroponics or coco coir, aim for a pH between 5.5 and 5.9. 

Maintaining the correct pH levels in your grow medium is the simplest way to avoid molybdenum deficiency in cannabis. 

Symptoms of molybdenum deficiency in cannabis plants 

When you have a molybdenum deficiency in marijuana plants, it’s easy to confuse it with a nitrogen deficiency, but there’s a way to distinguish between the two. 

The symptoms of a lack of nitrogen start at the base of your crops. A shortage of the molybdenum nutrient starts in the middle and moves upward

It’s a rare issue but requires swift treatment. Some of the signs of a molybdenum deficiency include:

  • Discoloration 
  • Miss-shapen leaves 
  • Stunted growth

Discoloration

Leaves from the middle part of the plant upward turn yellow prematurely, similarly to crops with a nitrogen deficiency. This discoloration spreads to the younger leaves, and in some cases, the yellow hue is also visible between the veins. 

Certain cannabis plants that lack the molybdenum nutrient also display reddish-pink leaves with dark-brown spots or sometimes purple hues. The color change begins at the edges of the leaves, with the blades losing the luscious green shade fairly quickly. 

This discoloration then spreads to the rest of the leaves’ surface. 

Discolored cannabis leaves
Discolored cannabis leaves

Misshapen leaves

Molybdenum deficiency in cannabis causes changes to the shape and texture of the leaves. 

At first, when the discoloration gets worse, the edges of the leaves begin to twist and curl. If they’re left untreated, they dehydrate further and start wilting

The leaf edges become crispy and dry. Ultimately, molybdenum deficiency in cannabis leaves makes them thick and brittle. They soon fall off. 

Carefully observe your weed crops for any of these signs. During the flowering stage, the leaves usually turn yellow as the crops develop. This isn’t a symptom of molybdenum deficiency in cannabis. 

Stunted growth

Molybdenum deficiency in cannabis prevents the crops from utilizing nutrients optimally. The lack of adequate nutrients restricts growth. 

How to fix molybdenum deficiency in marijuana plants

There are multiple ways to try and fix molybdenum deficiency in weed. Successful treatment depends on the stage of the issue and the symptoms being displayed. 

The most popular ways to overcome the problem are:

  • Flushing cannabis
  • Adjusting the pH
  • Pruning 
  • Nutrient application

There are also preventative measures you can put in place to avoid molybdenum deficiency in cannabis. Below, we discuss each of the possible solutions. 

Flushing cannabis

During the early stages of molybdenum deficiency in marijuana, you’ll notice a slight change in color around the edges of the leaves. The longer you leave it, the worse the symptoms get. Some growers try to minimize the damage by incorporating three solutions at once. 

These cultivators combine flushing, adjusting the pH, and pruning in hopes of restoring their plant’s health completely. To flush your weed, use plain water to wash away any remaining nutrients in the soil. 

The water forces your plants to use any nutrient reserves they have left. Flushing is a great way to avoid nutrient lockout

Many growers treat molybdenum deficiency in weed with a mixture of water and low levels of nutrients to replace the ones being washed away.

It’s a general rule to give the plant three times as much water as the size of the pot. If you have a three-gallon pot, you will flush with nine gallons of the liquid solution. 

You must remove the run-off water as soon as possible so the plant doesn’t absorb it.

Be sure to ventilate the area where you’re flushing your molybdenum deficiency in cannabis. Use a fan or two to help dry it out, as the excess hydration can make your plants droopy. 

You should also increase the space between your crops and the grow lights while your weed recovers.

Adjust the pH

If you have a molybdenum deficiency in your weed plants, your soil or substrate is most likely too acidic. Ensure that your water and nutrient solution stays at the correct pH, and allow the pot to dry completely before taking further action. 

Use a digital pH meter to test the alkalinity and maintain it at the recommended level. Some growers opt to use a measurement kit with drops, which are just as efficient for testing the pH.

Pruning

Regular pruning is essential for allowing adequate airflow and light exposure to your weed plants. After your crops have experienced a molybdenum nutrient deficiency, you’ll need to cut off any foliage that looks dry, damaged, or dead

If you need to prune the marijuana plants during the flowering stage, remove the leaf stems as well. This process will help you avoid bud mold.

Crops typically take 2-3 weeks to recover. Your harvest might be smaller than you initially anticipated, but salvaging a small yield is better than losing your whole crop because of a molybdenum deficiency in your cannabis.

Pruning weed
Pruning weed

Nutrient application 

When your crops are flushed, and the pH levels are within the optimal range, you can start supplementing your plants. Implement a feeding schedule to make sure that the crops get adequate nutrients

Molybdenum deficiency symptomHow to solve it
DiscolorationAdjust the pH
Misshapen leavesFlush cannabisAdjust the pH
Prune necrotic foliage
Stunted growthFlush cannabisAdjust the pH
Prune necrotic foliageImplement a nutrient feeding schedule
Healthy cannabis flowering
Healthy cannabis flowering

Conclusion

Cultivating cannabis is rewarding, but there can be some scary challenges that need urgent attention. Noticing that you have molybdenum deficiency in your cannabis crop falls into this category. 

There are various causes for this rare marijuana plant problem, but unbalanced pH levels are the most common. Cold temperatures also reduce the number of molybdenum nutrients available in the substrate. 

The signs of molybdenum deficiency in weed are quite easy to detect. The most common ones are discoloration and curled-up leaves

The secret to determining that you’re dealing with a lack of molybdenum is to check where the symptoms begin. Molybdenum deficiency in cannabis is typically visible from the middle of the plant upwards

If you manage to detect the molybdenum deficiency in your weed early, you can still save your crop by flushing it and correcting the pH levels. Now that you understand more about this marijuana plant issue, why not try growing your own?


About the author: Parker Curtis

Parker Curtis has around a decade of cannabis-growing experience, specialising in soil-less and hydro grows. He’s mastering outdoor, greenhouse, and indoor grows.